+ Documented the arguments.
+ Used BUCKET_SEARCH.
+ Vastly reduced nesting.
+ Changed the "*changed" argument to get set true whenever PER_CHANGED
  is called, i.e. whenever the bucket mutates.  The purpose of *changed
  wasn't documented, and its only use was in the BTree set routine, which
  is known to have at least one bug.  So it wasn't clear what the purpose
  of *changed was.  What it did do is get set true if and only if the
  key was found in the bucket and its value was replaced, and I couldn't
  imagine a plausible reason for why the BTree set routine could care
  about that alone (all other calls to _bucket_set pass NULL, so there
  were no other clues).
+ Fixed all places where error returns didn't finish the persistence
  dance.